Solve for $t$ : $15 = 6 + t$
Subtract $6$ from both sides: $ \begin{eqnarray} \\ \hphantom{15 {- 6}} &\hphantom{=}& \hphantom{t} \\ 15 &=& 6 + t \\ \\ {-6} && {-6} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 15 {- 6} &=& t \\ \end{eqnarray} $ Simplify. $ t = 9$